Halfback isn't hard to see (5)
I believe the answer is:
faint
'hard to see' is the definition.
(I've seen this in another clue)
'halfback isn't' is the wordplay.
'halfback' becomes 'f' (I am not sure about this - if you are sure you should believe this answer much more).
'isn't' becomes 'aint' (non-standard version of 'isn't').
'f'+'aint'='FAINT'
Can you help me to learn more?
(Other definitions for faint that I've seen before include "woozy" , "dull" , "Lacking clarity" , "Dim -- swoon" , "Barely perceptible" .)